It is well known to use a heat pump in order to utilize the heat energy stored in a heat reservoir, for instance in the earth surface. Due to the fact that it is very difficult to regulate the capacity of a compressor, the known device has the drawback that the compressor rather quickly after the start is adjusted to a certain temperature of condensation and compression independent of the demand for heating, such that the efficiency factor, i.e. the relation between the total heating effect and the consumed effect for the operation of the compressor does not become optimal.